This invention relates generally to the field of pressure sensing or pressure transducing devices. In particular, it relates to the class of such devices in which a pressure-responsive element is provided with solid-state strain gauges that produce an electrical output signal indicative of the pressure applied to the pressure-responsive element. More specifically, the present invention relates to a transducer of the aforementioned type which can simultaneously measure pressures in two different pressure ranges, or pressures of two different types (i.e., static pressure as well as differential pressure).
Fluid pressure transducers are commonplace in many industrial applications. Such transducers are employed, for example, to measure the differential pressure of a process fluid flowing through or past an orifice, nozzle, or other fluid flow discontinuity. It is also frequently desirable to measure the static pressure of the fluid at the same time. Instead of employing two separate transducers, the industry has widely adopted pressure-measuring instruments which combine, in a single instrument, both static and differential pressure transducing capabilities. An example of such an instrument is disclosed and claimed in U.S. Pat. No. 4,395,915 to Singh, commonly assigned with the present application. In the Singh device, differential pressure is measured by a pressure-responsive beam, while static pressure is measured by a separate, discrete diaphragm. Both the beam and the diaphragm are provided with piezoresistive strain gauges to produce appropriate pressure-indicative output signals.
Other instruments have been devised to measure, simultaneously, gauge pressure and absolute pressure. Examples of such devices are found in U.S. Pat. No. 4,222,277 to Kurtz et al. and U.S. Pat. No. 4,322,980 to Suzuki et al. Instruments of this type are characterized by a pair of discrete diaphragms formed in a single semiconductor wafer, each of the diaphragms being provided with its own set of strain gauges.
In U.S. Pat. No. 3,484,732 to Postma, a pressure measuring instrument is disclosed which is capable of pressure measurements in two different ranges. Low range pressures are measured by strain gauges provided on a hollow, rectangular frame, while strain gauges on a compression block measure high range pressures.
Thus, two basic types of dual-function pressure transducers have been provided by the prior art: The first type, exemplified by the Singh and Postma patents, uses separate and discrete pressure-responsive members to provide simultaneous static and differential pressure measurements (Singh) or dual-range pressure measurements (Postma). The second type, exemplified by the Kurtz et al. and the Suzuki et al. patents, uses a pair of pressure-responsive members formed from a single, unitary element (a wafer) to provide both gauge and absolute pressure measurements. This second type, however, is not totally satisfactory for applications involving measurements of pressures in lower pressure ranges, since lower pressures require relatively large diaphragms that can be fairly expensive to manufacture and cumbersome to package in a practical instrument. Thus, instruments designed to measure relatively small differential pressures, or low range static pressures, must have some other form of pressure-responsive element (i.e., a beam or a hollow frame) to provide such measurements.
Besides low pressure range capability, another advantage of beam-type instruments, of the construction exemplified by the aforementioned patent to Singh, is that isolation of the beam and its associated strain gauges from the measured fluid medium is provided. This isolation avoids problems of chemical incompatibility between the medium on the one hand, and the beam and/or its strain gauges on the other hand.
Thus, it would be a great advance in the art to provide a pressure measuring instrument which combines the fluid medium isolation and low pressure range capabilities of the beam-type of instrument, with the structural simplicity of the integral dual-diaphragm type of instrument.